A typical ultrasound probe consists of three basic parts: (1) a transducer package; (2) a multi-wire coaxial cable connecting the transducer to the rest of the ultrasound system; and (3) other miscellaneous mechanical hardware such as the probe housing, thermal/acoustic potting material and electrical shielding. The transducer package (sometimes referred to as a "pallet") is typically produced by stacking layers in sequence, as shown in FIG. 1.
First, a flexible printed circuit board 2 is bonded to a metal-coated rear face of a large piezoelectric ceramic block 4. A conductive foil 10 is bonded to a metal-coated front face of the piezoelectric ceramic block to provide a ground path for the ground electrodes of the final transducer array. Next, a first acoustic impedance matching layer 12 is bonded to the conductive foil 10. Optionally, a second acoustic impedance matching layer 14 having an acoustic impedance less than that of the first acoustic impedance matching layer 12 is bonded to the front face of the first matching layer 14.
The top portion of this stack is then "diced" by sawing vertical cuts, i.e., kerfs, from the rear face of the stack to a depth sufficient to divide the piezoelectric ceramic block into a multiplicity of separate side-by-side transducer elements. The kerfs produced by this dicing operation are depicted in FIG. 2. During dicing, the bus of the transducer flex circuit 2 (not shown in FIG. 2) is cut to form separate terminals and the metal-coated rear and front faces of the piezoelectric ceramic block are cut to form separate signal and ground electrodes respectively.
As shown in FIG. 1, the transducer stack also comprises a backing layer 8 made of suitable acoustical damping material having high acoustic losses, e.g., silver epoxy. This backing layer is coupled to the rear surface of the piezoelectric transducer elements to absorb ultrasonic waves that emerge from the back side of each element.
A known technique for electrically connecting the piezoelectric elements of a transducer array to a multi-wire coaxial cable is by a transducer flex circuit in which the conductive traces fan out, that is, a flex circuit having a plurality of etched conductive traces extending from a first terminal area to a second terminal area, the terminals in the first terminal area having a linear pitch greater than the linear pitch of the terminals in the second terminal area. The terminals in the first terminal areas are respectively connected to the individual wires of the coaxial cable. The terminals in the second terminal areas are respectively connected to the signal electrodes of the individual piezoelectric transducer elements.
Using this method, the resulting flex circuit can have signal runs which fan out so that the wires of a multi-wire coaxial cable, purchased from an outside vendor, can be attached directly. Since the circuit board is flexible, the wiring assembly can be folded to occupy a very small cross section. However, direct connection of the coaxial cable wires to the individual terminals of the flex circuit is a time-consuming and error-prone process. The primary difficulty is that each separate wire of the coaxial cable needs to be properly aligned and then bonded to a respective terminal of the flex circuit.